The invention relates to a piston pump which is intended in particular for use in a slip-controlled hydraulic vehicle brake system.
One such piston pump is known for instance from German Patent Disclosure DE 41 07 979 A1. The known piston pump has a cylindrical, boltlike piston, which is produced by metal-cutting machining with grooves (undercuts) and transverse bores as well as an axial blind bore. Producing the piston by metal-cutting machining is complicated and expensive. The piston is guided axially displaceably in a pump bore of a pump housing and can be driven to execute an axially reciprocating stroke motion by means of a rotationally drivable eccentric element. A positive displacement chamber of the piston pump adjoins the piston on one face end of the piston in the pump bore. This has the disadvantage of making the known piston pump long, requiring a large structural volume.
The piston pump according to the invention has a piston embodied as a hollow piston, whose interior forms a positive displacement chamber of the piston pump. Embodying the piston as a hollow piston, thus making it possible to shift the positive displacement chamber into the inside of the piston, makes a short piston pump of compact construction possible. A fixed internal part protrudes into an open side of the piston and defines the positive displacement chamber on one side of the piston. Because of the internal part protruding into the piston, an idle volume of the positive displacement chamber, or in other words a minimum volume of the positive displacement chamber at the end of a supply stroke, can be reduced virtually to zero, thus achieving high efficiency of the piston pump. The piston of the piston pump according to the invention is produced as a reshaped part by reshaping, such as deep drawing, cold heading, or extrusion. The piston is tubular or sleevelike, for instance, with an end wall that closes it on one side, all as one piece. At least before being assembled to the internal part, it has no undercuts and thus can be produced in a single operation quickly and simply by reshaping. To increase its wear resistance, the piston can be hardened; further machining or post-machining, such as fine-machining of its surface, is unnecessary. As a result, the piston can be produced inexpensively and quickly.
The piston is embodied as a stepped piston; that is, it is guided and sealed off in a pump bore in a pump housing at different diameters at two axially spaced-apart points. Embodying the piston as a stepped piston creates an annular chamber surrounding the piston, the volume of which alternatingly increases and decreases during the reciprocating stroke motion of the piston. During a supply stroke of the piston, in which the volume of the positive displacement chamber decreases, the volume of the annular chamber increases, and as a result fluid is aspirated by the piston pump into the annular chamber. During a return stroke of the piston, the volume of the annular chamber does decrease, but the volume of the positive displacement chamber increases to a greater extent than that by which the volume of the annular chamber decreases, and as a result the piston pump aspirates fluid. Because of the embodiment of its piston as a stepped piston, the piston pump according to the invention thus aspirates during both the supply stroke and the return stroke. This has the advantage that a volumetric aspiration flow is more uniform, which improves the aspiration performance and filling of the positive displacement chamber of the piston pump.
In one embodiment of the invention, the internal part has an undercut that the piston engages. The piston and the internal part form a preassembled structural unit; they are joined together in captive fashion. The advantage of this embodiment of the invention is the ease of handling of the preassembled structural unit, including the piston and the internal part, until its installation in a pump housing. Once the preassembled structural unit has been installed in the pump housing, the securing of the piston on the internal part is no longer needed, since the piston is secured on the internal part by the eccentric element. As a result, only slight demands are made in terms of the quality and strength of the connection of the piston and the internal part.
A check valve (inlet or outlet valve) is accommodated in the internal part; this valve controls a flow direction through the piston pump of fluid pumped by the piston pump. Accommodating the check valve in the internal part economizes on installation space and achieves a compact design of the piston pump.
In another refinement the check valve has a valve closing body, which is guided in a valve stroke direction by a valve closing body guide. At least one fluid passage is provided, through which when the check valve is open fluid can overflow the valve closing body. Because of the passage, it is possible to provide close tolerances for the valve closing body guidance with respect to the valve closing body. The close-tolerance valve closing body guide has the advantage of reducing noise, since it largely prevents sideward motions of the open valve closing body, lifted from a valve seat, that are caused by fluid bathing the valve closing body. In one feature of the invention, the valve closing body guide has guide ribs, which extend in the valve stroke direction and are formed for example in the form of a many-grooved internal profile. Flow grooves are located between the guide ribs and together these grooves have a large fluid passage area, so that with close-tolerance valve closing body guidance, the result is low flow resistance of the check valve.
The piston pump of the invention is intended in particular as a pump in a brake system of a vehicle and is used to control the pressure in wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS (for anti-lock brake system), TCS (traction control system), ESP (electronic stability program) and EHB (electrohydraulic brake system) are used for such brake systems. In the brake system, the pump serves for instance to return brake fluid from a wheel brake cylinder or a plurality of wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into a wheel brake cylinder or a plurality of wheel brake cylinders (TCS or ESP or EHB). The pump is required in a brake system with wheel slip control (ABS or TCS) and/or a brake system serving as a steering aid (ESP) and/or an electrohydraulic brake system (EHB). With wheel slip control (ABS or TCS), locking of the vehicle wheels during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (TCS) can for instance be prevented. In a brake system serving as a steering aid (ESP), a brake pressure is built up in one or more wheel brake cylinders independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from breaking out of the track desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB), in which the pump pumps the brake fluid into the wheel brake cylinder or wheel brake cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump is used to fill a reservoir of the brake system.